FAQ/TIPS

The term aggregate refers generally to all types of sands, gravels and crushed stone which are used in a variety of construction applications and are products of a pit or quarry.

Generally, a pit contains sand and gravel that is directly excavated, screened, and transported. A pit can produce some products that consist of round stones, whereas products from a quarry are typically crushed which can lead to angular shaped edges. A quarry commonly contains rock that must be blasted first before it can be processed through crushing and screening into smaller sized products.

Sand and Gravel are key ingredients that are used to build roads, sidewalks, schools, factories, offices, parking lots, driveways, basements, walls, roofs, gardens, walkways, paths, recreation centers, sports fields, golf courses sand traps, landscape rock, and hiking trails, among many other applications. Industrial sands can be used in the process of making glass, for grit for roofing paper, and as additives to paint and stucco.

Sand and gravel are naturally occurring, typically in pits, while crushed stone is a product of blasting, crushing and processing rock, typically in a quarry.

Sand and gravel are found in certain geological settings around Texas. They are not found everywhere. They were formed when rock was ground up through the movement and melting of glaciers during the last ice age which ended tens of thousands of years ago. They are also formed as a transportation process of rocks and minerals being moved down river systems, being deposited over thousands of years along bends where water velocity is reduced. Deposits are often found in and near river deltas and at widenings in rivers where flow slows and deposits fall out.

There are many steps that the raw product goes through before it is sold to the end user. A pile of aggregate does not have only one grain size but it has a distribution of sizes, ranging from small sands to 6 inch and higher cobbles.

The first operation is usually a jaw crusher; two plates are forced together on a rotating basis which breaks up large pieces of rock, found in almost all deposits. Conveyor belts take the product to screening, followed by recirculation to a cone crusher for larger particles.

Screening and classifying separates particles of similar size in one pile. This is accomplished through the use of mechanically vibrated screens, or sieves, stacked one on top of the other.

Larger particles which remain are sent back to the cone crusher, which takes the recirculated product and reduces the size as it falls through a rotating core to break the rock. The product is then sent back to the screen. This recirculation occurs over and over for any larger sizes remaining at the top of the screen. In a rock quarry, there is more recirculation through the cone crusher.

For various industrial applications there are different standards that dictate the series of sieves to be used and what percentage of material must be in each sieve to be a given size (e.g. 1/2″ or 1/4″). Product passing through to the bottom of the screens is conveyed into piles or a surge bin, which then fills trucks for transport.

A washing cycle may also be included, depending on the type of product. Washed, or clear, gravel will have any fines (dust, clays, fine grained) removed.

A base product is made up of a crushed or natural coarse rock and sand that is well graded to produce, when compacted, a firm stabilized sub­grade for streets, highways or even concrete slabs.

There are many products that will provide a good drainage material. The material to be used is dependent on how much water flow is anticipated. If the water flow is limited, a 1/4″ size rock can be used. If the water flow is higher, a larger size rock with a high void ratio (the open space between compacted rock) such as a 3/4″ to 1″ size rock is needed.

Riprap is comprised of large pieces of rock (usually between 6 and 30 inches in diameter) which have undergone only primary crushing and sizing. Riprap is used to stabilize slopes and shorelines as well as to construct erosion-control structures.